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          <th colspan="3" align="center">Db::compact()</th>
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                The Db Handle
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    <div class="sect1" lang="en" xml:lang="en">
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          <div>
            <h2 class="title" style="clear: both"><a id="dbcompact"></a>Db::compact()</h2>
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      <pre class="programlisting">#include &lt;db_cxx.h&gt;
 
int
Db::compact(DbTxn *txnid,
    Dbt *start, Dbt *stop, DB_COMPACT *c_data, u_int32_t flags, Dbt *end);</pre>
      <p>
         The <code class="methodname">Db::compact()</code> method compacts Btree,
         Hash, and Recno access method databases, and optionally returns
         unused Btree, Hash or Recno database pages to the underlying
         filesystem.
    </p>
      <p>
         The <code class="methodname">Db::compact()</code> <span>
            
            <span>
                method either returns a non-zero error value or throws an
                exception that encapsulates a non-zero error value on
                failure, and returns 0 on success.
            </span>
        </span>
    </p>
      <div class="sect2" lang="en" xml:lang="en">
        <div class="titlepage">
          <div>
            <div>
              <h3 class="title"><a id="idm448944"></a>Parameters</h3>
            </div>
          </div>
        </div>
        <div class="sect3" lang="en" xml:lang="en">
          <div class="titlepage">
            <div>
              <div>
                <h4 class="title"><a id="idm165544"></a>txnid</h4>
              </div>
            </div>
          </div>
          <p>
                  If the operation is part of an application-specified transaction, the
                  <span class="bold"><strong>txnid</strong></span> parameter is a transaction
                  handle returned from <a class="xref" href="txnbegin.html" title="DbEnv::txn_begin()">DbEnv::txn_begin()</a>; 
                  if the operation is part of a Berkeley DB Concurrent Data Store group, the
                  <span class="bold"><strong>txnid</strong></span> parameter is a handle returned
                  from <a class="xref" href="envcdsgroup_begin.html" title="DbEnv::cdsgroup_begin()">DbEnv::cdsgroup_begin()</a>;
                  otherwise NULL.
              </p>
          <p>
                  If a transaction handle is supplied to this method, then the operation is
                  performed using that transaction. In this event, large sections of the
                  tree may be locked during the course of the transaction.
              </p>
          <p>
                  If no transaction handle is specified, but the operation occurs
                  in a transactional database, the operation will be implicitly
                  transaction protected using multiple transactions.  These transactions
                  will be periodically committed to avoid locking large sections of the
                  tree.  Any deadlocks encountered cause the compaction operation to
                  be retried from the point of the last transaction commit.
             </p>
        </div>
        <div class="sect3" lang="en" xml:lang="en">
          <div class="titlepage">
            <div>
              <div>
                <h4 class="title"><a id="idm1372928"></a>start</h4>
              </div>
            </div>
          </div>
          <p>
                  If non-NULL, the <span class="bold"><strong>start</strong></span>
                  parameter is the starting point for compaction.  For a
                  Btree or Recno database, compaction will start at the
                  smallest key greater than or equal to the specified key.
                  For a Hash database, the compaction will start in the
                  bucket specified by the integer stored in the key.  If
                  NULL, compaction will start at the beginning of the
                  database.
             </p>
        </div>
        <div class="sect3" lang="en" xml:lang="en">
          <div class="titlepage">
            <div>
              <div>
                <h4 class="title"><a id="idm1085136"></a>stop</h4>
              </div>
            </div>
          </div>
          <p>
                  If non-NULL, the <span class="bold"><strong>stop</strong></span>
                  parameter is the stopping point for compaction.  For a
                  Btree or Recno database, compaction will stop at the page
                  with the smallest key greater than the specified key.
                  For a Hash database, compaction will stop in the bucket
                  specified by the integer stored in the key.  If NULL,
                  compaction will stop at the end of the database.
             </p>
        </div>
        <div class="sect3" lang="en" xml:lang="en">
          <div class="titlepage">
            <div>
              <div>
                <h4 class="title"><a id="idm1896352"></a>c_data</h4>
              </div>
            </div>
          </div>
          <p>
                  If non-NULL, the <span class="bold"><strong>c_data</strong></span> parameter
                  contains additional compaction configuration parameters, and returns
                  compaction operation statistics, in a structure of type 
                  <code class="literal">DB_COMPACT</code>.
             </p>
          <p>
                  The following input configuration fields are available from the
                  <code class="literal">DB_COMPACT</code> structure:
             </p>
          <div class="itemizedlist">
            <ul type="disc">
              <li>
                <p>
                        <code class="literal">int compact_fillpercent;</code>
                    </p>
                <p>
                        If non-zero, this provides the goal for filling
                        pages, specified as a percentage between 1 and 100.
                        Any page in the database not at or above this
                        percentage full will be considered for compaction.
                        The default behavior is to consider every page for
                        compaction, regardless of its page fill percentage.
                    </p>
              </li>
              <li>
                <p>
                        <code class="literal">int compact_pages;</code>
                    </p>
                <p>
                        If non-zero, the call will return after the
                        specified number of pages have been freed, or no
                        more pages can be freed.
                    </p>
              </li>
              <li>
                <p>
                        <code class="literal">db_timeout_t compact_timeout;</code>
                    </p>
                <p>
                        If non-zero, and no 
                        <span class="bold"><strong>txnid</strong></span> parameter was
                        specified, this parameter identifies the lock
                        timeout used for implicit transactions, in
                        microseconds.
                    </p>
              </li>
            </ul>
          </div>
          <p>
                The following output statistics fields are available from the
                <code class="literal">DB_COMPACT</code> structure:
            </p>
          <div class="itemizedlist">
            <ul type="disc">
              <li>
                <p>
                        <code class="literal">u_int32_t compact_deadlock;</code>
                    </p>
                <p>
                        An output statistics parameter: if no 
                        <span class="bold"><strong>txnid</strong></span> parameter was
                        specified, the number of deadlocks which occurred.
                    </p>
              </li>
              <li>
                <p>
                        <code class="literal">u_int32_t compact_pages_examine;</code>
                    </p>
                <p>
                        An output statistics parameter: the number of
                        database pages reviewed during the compaction
                        phase.
                    </p>
              </li>
              <li>
                <p>
                      <code class="literal">u_int32_t compact_empty_buckets;</code>
                  </p>
                <p>
                       An output statistics parameter: the number of empty
                       hash buckets that were found the compaction phase.
                  </p>
              </li>
              <li>
                <p>
                      <code class="literal">u_int32_t compact_pages_free;</code>
                  </p>
                <p>
                       An output statistics parameter: the number of
                       database pages freed during the compaction phase.
                  </p>
              </li>
              <li>
                <p>
                      <code class="literal">u_int32_t compact_levels;</code>
                  </p>
                <p>
                       An output statistics parameter: the number of levels
                       removed from the Btree or Recno database during the
                       compaction phase.
                  </p>
              </li>
              <li>
                <p>
                      <code class="literal">u_int32_t compact_pages_truncated;</code>
                  </p>
                <p>
                       An output statistics parameter: the number of
                       database pages returned to the filesystem.
                  </p>
              </li>
            </ul>
          </div>
        </div>
        <div class="sect3" lang="en" xml:lang="en">
          <div class="titlepage">
            <div>
              <div>
                <h4 class="title"><a id="idp4593856"></a>flags</h4>
              </div>
            </div>
          </div>
          <p>
                  The <span class="bold"><strong>flags</strong></span> parameter must be set to 0
                  or one of the following values:
             </p>
          <div class="itemizedlist">
            <ul type="disc">
              <li>
                <p><a id="compact_DB_FREELIST_ONLY"></a>
                         <code class="literal">DB_FREELIST_ONLY</code>
                   </p>
                <p>
                        Do no page compaction, only returning pages to the
                        filesystem that are already free and at the end of
                        the file.
                   </p>
              </li>
              <li>
                <p><a id="compact_DB_FREE_SPACE"></a>
                         <code class="literal">DB_FREE_SPACE</code>
                   </p>
                <p>
                        Return pages to the filesystem when possible. If
                        this flag is not specified, pages emptied as a
                        result of compaction will be placed on the free
                        list for re-use, but never returned to the
                        filesystem.
                   </p>
                <p>
                        Note that only pages at the end of a file can be
                        returned to the filesystem.  Because of the
                        one-pass nature of the compaction algorithm, any
                        unemptied page near the end of the file inhibits
                        returning pages to the file system.  A repeated
                        call to the <code class="methodname">Db::compact()</code>
                        method with a low 
                        <span class="bold"><strong>compact_fillpercent</strong></span> 
                        may be used to return pages in this case.
                   </p>
              </li>
            </ul>
          </div>
        </div>
        <div class="sect3" lang="en" xml:lang="en">
          <div class="titlepage">
            <div>
              <div>
                <h4 class="title"><a id="idp3774880"></a>end</h4>
              </div>
            </div>
          </div>
          <p>
                  If non-NULL, the <span class="bold"><strong>end</strong></span>
                  parameter will be filled with the database key marking
                  the end of the compaction operation in a Btree or Recno
                  database.  This is generally the first key of the page
                  where the operation stopped.  For a Hash database, this
                  will hold the integer value representing which bucket the
                  compaction stopped in.
             </p>
        </div>
      </div>
      <div class="sect2" lang="en" xml:lang="en">
        <div class="titlepage">
          <div>
            <div>
              <h3 class="title"><a id="idp3106064"></a>Errors</h3>
            </div>
          </div>
        </div>
        <p>
             The <code class="methodname">Db::compact()</code> <span>
            
            <span>
                method may fail and throw a <a class="link" href="dbexception.html" title="Chapter 6. The DbException Class">DbException</a> 
                exception, encapsulating one of the following non-zero errors, or return one
                of the following non-zero errors:
            </span>
        </span>
        </p>
        <div class="sect3" lang="en" xml:lang="en">
          <div class="titlepage">
            <div>
              <div>
                <h4 class="title"><a id="idp3880208"></a><span>DbDeadlockException or </span>DB_LOCK_DEADLOCK</h4>
              </div>
            </div>
          </div>
          <p>
                A transactional database environment operation was selected to resolve
                a deadlock.
            </p>
          <p>
                <a class="xref" href="dbdeadlock.html" title="DbDeadlockException">DbDeadlockException</a> is thrown if
                your Berkeley DB API is configured to throw exceptions.
                Otherwise, <code class="literal">DB_LOCK_DEADLOCK</code> is returned.
            </p>
        </div>
        <div class="sect3" lang="en" xml:lang="en">
          <div class="titlepage">
            <div>
              <div>
                <h4 class="title"><a id="idp2656000"></a><span>DbLockNotGrantedException or </span>DB_LOCK_NOTGRANTED</h4>
              </div>
            </div>
          </div>
          <p>
                A Berkeley DB Concurrent Data Store database environment configured
                for lock timeouts was unable to grant a lock in the allowed time.
            </p>
          <p>
                You attempted to open a database handle that is configured
                for no waiting exclusive locking, but the exclusive lock could not be
                immediately obtained. See
                <a class="xref" href="dbset_lk_exclusive.html" title="Db::set_lk_exclusive()">Db::set_lk_exclusive()</a>
                for more information.
            </p>
          <p>
                <a class="xref" href="dblocknotgranted.html" title="DbLockNotGrantedException">DbLockNotGrantedException</a> is thrown if
                your Berkeley DB API is configured to throw exceptions.
                Otherwise, <code class="literal">DB_LOCK_NOTGRANTED</code> is returned.
            </p>
        </div>
        <div class="sect3" lang="en" xml:lang="en">
          <div class="titlepage">
            <div>
              <div>
                <h4 class="title"><a id="idp3056480"></a> <span>DbRepHandleDeadException or</span> DB_REP_HANDLE_DEAD</h4>
              </div>
            </div>
          </div>
          <p>
                When a client synchronizes with the master, it is possible for committed
                transactions to be rolled back. This invalidates all  the database and cursor
                handles opened in the replication environment. Once this occurs, an attempt to use
                such a handle will 
                <span>
                    throw a <a class="xref" href="dbrephandledead.html" title="DbRepHandleDeadException">DbRepHandleDeadException</a> (if
                    your application is configured to throw exceptions), or 
                </span>
                return <code class="literal">DB_REP_HANDLE_DEAD</code>.
                The application will need to discard the handle and open a new one in order to
                continue processing.
            </p>
        </div>
        <div class="sect3" lang="en" xml:lang="en">
          <div class="titlepage">
            <div>
              <div>
                <h4 class="title"><a id="idp3010448"></a><span>DbDeadlockException or </span>DB_REP_LOCKOUT</h4>
              </div>
            </div>
          </div>
          <p>
                The operation was blocked by client/master synchronization.
            </p>
          <p>
                <a class="xref" href="dbdeadlock.html" title="DbDeadlockException">DbDeadlockException</a> is thrown if
                your Berkeley DB API is configured to throw exceptions.
                Otherwise, <code class="literal">DB_REP_LOCKOUT</code> is returned.
            </p>
        </div>
        <div class="sect3" lang="en" xml:lang="en">
          <div class="titlepage">
            <div>
              <div>
                <h4 class="title"><a id="idp1673560"></a>EACCES</h4>
              </div>
            </div>
          </div>
          <p>
                An attempt was made to modify a read-only database.
            </p>
        </div>
        <div class="sect3" lang="en" xml:lang="en">
          <div class="titlepage">
            <div>
              <div>
                <h4 class="title"><a id="idp1696840"></a>EINVAL</h4>
              </div>
            </div>
          </div>
          <p>
                An invalid flag value or parameter was specified.
            </p>
        </div>
      </div>
      <div class="sect2" lang="en" xml:lang="en">
        <div class="titlepage">
          <div>
            <div>
              <h3 class="title"><a id="idp3838680"></a>Class</h3>
            </div>
          </div>
        </div>
        <p>
            <a class="link" href="db.html" title="Chapter 2.  The Db Handle">Db</a>  
        </p>
      </div>
      <div class="sect2" lang="en" xml:lang="en">
        <div class="titlepage">
          <div>
            <div>
              <h3 class="title"><a id="idp1905120"></a>See Also</h3>
            </div>
          </div>
        </div>
        <p>
             <a class="xref" href="db.html#dblist" title="Database and Related Methods">Database and Related Methods</a> 
        </p>
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